Method and apparatus for determining the intensity distribution of a radiation field

ABSTRACT

The invention relates to a method and apparatus for determining the intensity distribution of a radiation field. In the method, the ionization caused by the radiation field is detected by parallel ionization detector planes arranged in an ionization chamber, two of which planes are formed of series of wires determining the position of an ionization event in the X and Y directions, which detector planes provide the X and Y co-ordinates X 1 , Y 1  of the ionization event. The ionization event is created by photonuclear reaction products arising from the radiation.

TECHNICAL FIELD

The present invention relates to a method and apparatus for determiningthe intensity distribution of a radiation field, which apparatusincludes at least two parallel ionization detector planes arranged in anionization chamber, which are arranged to be formed of a series of wiresdetermining the location of an ionization event in an X and an Ydirection, and in which the detector planes are connected to measurementelectronics, in such a way that the said two detector planes arearranged to provide the X and Y co-ordinates of the ionization event.

BACKGROUND OF THE INVENTION

The regular and sufficiently accurate and careful performance ofmeasurement and other quality-control measures for patient-careequipment, is the foundation of reliable radiotherapy. Together withchanges in the conditions and demands of this work, the development ofexternal radiotherapy methods, accelerators, and equipment relating tothe radiotherapy chain has brought new challenges for those working withradiotherapy.

It is difficult for hospital physicists to carry out their duties withinthe constraints of working time and existing personnel resources. Thecomplexity of the measuring devices and the preparations required bymeasurements mean that making quality control measurements ofpatient-care equipment using present equipment is also time consuming.Making measurements takes up the entire working time of hospitalphysicists, leaving them no time to carry out any other tasks. Inaddition, the measurements specified in a quality-control program forradiotherapy equipment cannot be made during normal working time, as theequipment is being used for therapy at that time.

Besides changing demands, increased pressure to develop measuringequipment that would accelerate and simplify the tasks of hospitalphysicists has also arisen from the development of data management anddatabase systems.

One equipment solution according to the state of the art, presently usedfor quality-control measurements of patient-care equipment, isrepresented by the so-called water phantom. A water phantom comprises awater-filled plexiglass box, inside which ionization detectors measuringthe intensity of the radiation field on an X-Y plane are moved. Theionization detectors, of which there are typically 1–24, are arranged ina comb shape.

The planar movement of the detectors can be limited to either the X orthe Y direction. The plane measurement is repeated at different depthsin the Z direction. Due to the small number of detectors, the positionmeasurement is accurate to only the order of a few centimeters. Anyincrease in the number of detectors will significantly raise the priceof the device and the amount of complex electronics required.

A measurement made using the detector in question may take up to severalhours, during which time the ambient temperature, among other factors,can vary, simultaneously altering the gas pressure in the ionizationchamber of the detectors. It is difficult to compensate later for themeasurement error that this creates and which in any event reduces thereliability of the measurement.

In addition to the above, due to the measurement principle of sweepingthe radiation field of the detectors, the water phantom according to thestate of the art cannot measure the intensity distribution of a fieldindependently of time. Practically all new accelerators aretime-dependent, so-called dynamic-field accelerators, making itextremely desirable to also be able to determine the intensitydistribution of the field.

A second device representing the state of the art is a plane detectorcomprising typically less than 10 ionization chambers, which is used inhigh-speed quality assurance measurements to check the stability,evenness, and symmetry of a radiation field. The position resolutionability of these devices is poor, being several centimeters, and theyare unable to measure small variations in position in a dynamic field.

On the basis of the state of the art, it can be further asserted thateven the latest detector models have considerably lagged far behind thedevelopment of accelerators and of other measuring and analysis devices.The fact that known devices are out of date can also be seen in theirquite simple user interfaces.

Yet a third device impinging on the state of the art is disclosed inU.S. Pat. No. 4,485,307. The device is intended for radioisotopediagnostics within an organ. In it, the XY-plane detectors set in a gasplenum are formed from two cathode layers. The cathode layer is formedof wires running in one direction and set at equal intervals to eachother. The orientation of the wires of the layers is arranged such thatthe XY position of radiation can be determined on the basis of them.However, apparatus of this type has only a poor ability to determine inreal time the shape of the radiation fields of modern high-energyradiotherapy devices creating dynamic fields. Further, radioisotopediagnostics have an operating environment that is, in terms of theenergetics and intensity of the radiation field for instance, of atotally different order of magnitude to that of patient-care equipment,thus excluding the use of the apparatus disclosed in the publication in,for example, an accelerator environment.

SUMMARY OF THE INVENTION

The present invention is intended to create an entirely new type ofapparatus suitable for monitoring the quality of patient-care equipment,which can be used to make measurements of both static and dynamicradiation fields easily and rapidly, and which eliminates thepossibility of human error in dosimetric measurement. The invention alsorelates to a method for use in the apparatus, which permits the shape ofa high-energy radiation field to be determined using the operatingprinciple of an ionization chamber. The characteristic features of themethod for determining the intensity distribution of a radiation field,in which the ionization caused by the radiation field is detected bymeans of parallel ionization detector planes arranged in an ionizationchamber, two of which planes are formed of series of wires determiningthe position of the ionization event in the X and Y directions, whichdetector planes provide the X and Y co-ordinates X₁, Y₁ of theionization event, is characterized in that the ionization event iscreated by means of the photonuclear reaction products arising from theradiation.

An apparatus for determining the intensity distribution of a radiationfield, which apparatus includes at least two parallel ionizationdetector planes arranged in an ionization chamber, which are arranged tobe formed of series of wires determining the position of an ionizationevent in the X and Y directions and that they are arranged to providethe X and Y co-ordinates X₁, Y₁ of the ionization event, ischaracterized in that, in addition, a photoreaction converter is fittedto the ionization chamber, in order to achieve indirect detection in theapparatus.

In the method according to the invention, photonuclear reactions arisingfrom radiation are used to determine the shape of a radiation field. Theapparatus includes a photonuclear reaction converter. According to onepreferred embodiment, the direction of travel of the photonuclearreaction products is collimated. This improves the position precision ofthe apparatus. According to another preferred embodiment, thephotonuclear reaction converter can be formed of layers of one orseveral substances, making it possible to change the energy area and theintensity sensitivity being investigated with the apparatus.

The apparatus according to the invention will, among other things, makethe work of hospital personnel significantly more meaningful andmotivated, because the measurement event itself will be considerablyfaster than when using presently known apparatuses.

Compared to the state of the art, by using the apparatus according tothe invention a real-time image of the radiation field is createdrapidly with a single measurement. This also permits the timedependencies of the dynamic fields achieved using present patient-caretechnology to be determined and allows the apparatus to be neutral tochanges taking place in the environment. The measurement is thusconsiderably more reliable and its position precision is better than inthe state of the art. In addition to this, the shape of the field can bechanged in real time, by adjusting the accelerator and its parameters.

The apparatus according to the invention is unaffected by radiationdamage and achieves the additional advantages of, among other things, asmall shielding effect and easily arranged measurement of even extensiveradiation fields. It is also characterized by the simplicity andcheapness of its electronics, in comparison to apparatuses based on thestate of the art. The other characteristic features of the apparatusaccording to the invention will become apparent from the accompanyingclaims.

These and other features and advantages of the invention will be morefully understood from the following detailed description of theinvention taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of one apparatus according to theinvention,

FIG. 2 shows a cut-off cross-section of the apparatus shown in FIG. 1,

FIG. 3 shows a schematic diagram of an improved apparatus according tothe invention,

FIG. 4 shows schematic diagram of the electronics of the improvedapparatus according to the invention shown in FIG. 3,

FIG. 5 shows a schematic diagram of a second embodiment of thephotonuclear reaction converter, and

FIG. 6 shows neutron interaction areas in some possible photonuclearreaction-converter materials.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show the simplified operating principle of one embodimentof the apparatus according to the invention. The case of the apparatusis formed by an ionization chamber 10, which is, for example, 30-cm longand 1-cm high. The ionization chamber 10 is filled with an ionizing gas,such as n-pentane, as a negative pressure. Inert gases, carbon hydrides,and generally compounds of them can be used as the filler gas of thechamber 10. The use of a negative pressure (for example, 1–6 Torr)results in the ionization chamber 10 having, in reality, slightlyrounded sides.

In the embodiment shown in FIGS. 1 and 2, a mesh-like lo detector plane12, which acts as a cathode plane rejecting electrons and attractingpositive gas molecules and ions, and is formed from wires 12.1, 12.2,for example, of wolfram (wire diameter 100 μm), connected to thenegative potential 21.1 of a power supply 21, is located in theionization chamber 10. A signal wire 13.3 is connected to the cathodeplane 12, for leading the current impulse t, which arises in theionization chamber 10 due to the ionization caused by the radiation, tothe measurement electronics 18.

Detector planes 11.1 and 11.2, which act as anodes and attractelectrons, and are connected to the power source 21, are arrangedrelative to the cathode plane 12, for example, a short distance (e.g.,7–8 mm) above and beneath it and in parallel with the cathode plane 12.The potential 21.2 of the power supply 21 is of the opposite sign tothat of the potential 21.1 connected to the cathode plane 12.

The anode planes 11.1, 11.2 are formed of parallel wires 14.1, 14.2placed next to each other a constant distance apart, the material ofwhich can be, for example, gold-plated wolfram, and the diameter ofwhich is 20–40 μm. The anode planes 11.1, 11.2 are preferably arrangedin the ionization chamber 10 in such a way that the wires 14.1 of anodeplane 11.1 are oriented at right angles to the wires 14.2 of anode plane11.2, so that detector plane 11.1 determines the X direction anddetector plane 11.2 determines the Y direction. In the apparatusaccording to the example, the number of wires 14.1, 14.2 in eachdetector plane 11.1, 11.2 is 100, so that together they form 10000intersection points. The distance between the wires can be, for example,1–10 mm, preferably 2–7 mm.

The cathode plane 12 and the anode planes 11.1, 11.2 set on either sideof it are arranged in the ionization chamber 10 to be parallel to thewall 10.1 of the ionization chamber 10 that essentially faces theincoming direction of the radiation field 23.

According to one preferred embodiment, the wires 14.1, 14.2 forming theanode planes 11.1, 11.2 are connected to each other at one end of theplanes 11.1, 11.2 by delay elements 15, set at right angles to the wires14.1, 14.2. The delay elements 15 form sequentially connected delaylines 17.1, 17.2 of the anode planes 11.1, 11.2. The delay elements 15are characterized by the fact that the speed of travel of the charge inthem is known.

The delay lines 17.1, 17.2 shown in FIGS. 1 and 2 depict them at aschematic level. In reality, the delay elements 15 and the delay lines17.1, 17.2 formed by them are implemented by a series of adjacent wires14.1, 14.2 forming wire series, for each one of which delay elements 15are arranged by means of integrated semiconductor components (notshown), and the semiconductor components, set consecutively, formingfunctionally the delay lines 17.1, 17.2 shown in FIGS. 1 and 2.

According to one preferred embodiment shown in FIGS. 1 and 2, the firstend of the delay lines 17.1, 17.2 is arranged to terminate at the wire14.1′, 14.2′ nearest the edge of the anode plane 11.1, 11.2. Startingfrom the wire nearest the edge 14.1′, 14.2′, the delay elements 15connect the ends of the wires 14.1, 14.2 of the anode plane 11.1, 11.2to each other to the wires 14.1*, 14.2* of the opposite edge of theanode plane 11.1, 11.2, so that the current pulses X₁, Y₁ arising in theionization 10 as a result of the radiation are led by the delay lines17.1, 17.2 to the measurement electronics 18.

An embodiment is also possible, in which a separate line to themeasurement electronics is arranged from each wire 14.1, 14.2, but thisimplementation will substantially complicate the apparatus.

Because the operation of the apparatus according to the invention isbased on the observation of the ionization caused in a gas by thehigh-energy radiation emitted by a patient-care device, certainresolution problems are caused by the high energy and powerful intensityof the radiation, the background to which will next be examined ingreater detail.

When using the accelerators (for example, the Linac accelerators)presently used in patient care, the typical radiation creates about2*10¹⁵ pairs of ions each second in 1 kg of air. This corresponds toabout 2*10⁹ pairs of ions arising in cm³ of air in NTP conditions eachsecond (density of air 1,293 mg/cm³).

The amount of radiation travelling as far as the detector can beestimated, for example, by using the Klein-Nishina formula to calculatethe angle distribution of the scattered photons at various levels ofenergy. It will then be observed that more than 90% of the photons at 10MeV scatter at an angle of less than 10 degrees relative to theirdirection of arrival. Assuming that the scattering is on average, forexample, in the order of two degrees, the energy released in thescattering of one photon is 120 keV. This corresponds to nearly 600000photon interactions each second in 1 cm³ of air in NTP (the same photononly interacts once).

The ionization chamber 10 according to the invention has an effectivevolume, in the case according to the example (about 2,5 liters), that ismore than one thousand times that used in the above example of thecalculation. This represents a frequency of more than 600 MHz, whichexceeds the 1 MHz limit of which the measurement electronics 18 of theapparatus according to the example is capable.

The frequency can be reduced by lowering the intensity of theaccelerator, by using a gas with poorer ionization, by reducing thepressure of the gas or the volume of the ionization chamber 10, etc.However, it is then possible that a sufficient ignition voltage willfail to be achieved by the radiation, so that the probability of theelectrons colliding with each other will become too low and the electronavalanche phenomenon essential to the operation of the ionizationchamber 10 will not be created.

In the apparatus according to the invention, this problem is solved byexploiting photonuclear reactions. In that case, the energetic photonemitted in the radiation performed by the patient-care device interactswith the heavy nucleus to create a nuclear reaction.

According to the method of the invention, in order to achieve aphotonuclear reaction, a photonuclear reaction converter 16, in the formof, for example, a thin uranium or beryllium layer, is arranged on theinside of the wall 10.1, which faces the direction of the radiationfield 23, of the ionization chamber 10 according to the invention andshown in FIGS. 1 and 2. The uranium is preferably used for a radiationof 15–20 MeV. When using uranium, the thickness of the photonuclearreaction converter 16 is 0,1–100 μm, preferably 3–15 μm, and depends onthe surfacing material used. When using beryllium, the thickness may beeven greater. It is known that the cross-section area of photonuclearreactions varies from a few tens of millibarns (1 mb=10⁻³ b=10⁻²⁴ cm) tomore than 300 millibarns, at typical accelerator energies. Thephotonuclear reaction recoils, heavy nuclei of uranium and alphaparticles from beryllium, ionize the gas in the chamber 10 verypowerfully. Besides the heavy nucleus, the uranium reaction creates oneor more light particles, such as neutrons (so-called photoneutronreaction) or alternatively two medium-heavy nuclei (photofission).

A collimator 19 is installed in connection with the photonuclearreaction converter 16, to control the direction of travel of thephotonuclear reaction products. By means of the collimator 19, the pathof the reaction products created is made in one direction straightdownwards towards the anode plane 11.1, 11.2, according to the setcriteria of the characteristic parameters of the collimator 19. Thisprevents the distorting effect on the ionizing position data caused bylaterally-directed reaction products in the ionization chamber 10.

The calculation frequency of the apparatus is now determined secondarilyfrom the number of photonuclear reaction products, instead of primarilyfrom the ionization caused by the radiation in the gas. This can beestimated, if the number of target atoms per cm² in the photonuclearreaction converter 16, the dependence of the cross-section area of aphotoneutron reaction on the photon energy, and the photon flux persecond are known.

The photon flux can be estimated using the law of absorption, in theutilization of which must also be estimated the photon flux passedstraight through the gas without interacting with it and the originalflux of the photons. In addition, the parameters required whenestimating the photon flux include the total absorption coefficient(cm²/g) of the gas (for instance, oxygen) in the ionization chamber 10,the surface density of the gas (=1,293*10⁻³ g/cm²), and the thickness ofthe gas layer (=1 cm).

It is known that, for example, when the absorption coefficient of oxygengas O₂ is examined as a function of photon energy, the absorptiontypically takes place at the accelerator energies used in patient carealmost entirely through Compton scattering (incoherent scattering) andonly at energies approaching 20 MeV does pairing production begin bebecome a significant interaction mechanism.

According to the previous examination, 600 000 Compton scatterings takeplace each second in cm³ (NTP), in other words, the difference betweenthe original flux of the photons and the flux that has passed straightthrough the gas without interacting with it is 6*10⁵. By placing thisestimated total absorption coefficient (˜2*10²) of the gas and the givensurface density and surface layer thickness of the gas in the absorptionlaw and solving it in relation to the original photon flux, the resultobtained is 2,3*10¹⁰ s⁻¹. The result can be regarded as an estimate ofthe photon flux per second, in which case the number of reactionsarising is obtained as about 10 000 reactions per second, for example,with a 1 mg/cm² (0,5 μm)-thick uranium membrane and a radiation of 10MeV. (cross-section area ˜200 mb), which is already clearly within thecalculation capacity (1 MHz) of the apparatus according to theinvention.

It should be noted that in the calculation example given, the aim isonly to roughly estimate the order of magnitude of the number ofreactions. In reality, the retardation radiation spectrum is continuous,and thus includes all the photon energies up to the maximum energy ofthe accelerator. The 10 MeV energy used in the estimation and thecorresponding cross-section area are thus ‘average values’.

By deposition, it is possible to manufacture very thin uranium layers,so that the number of reactions can, if necessary, be reduced almostarbitrarily. However, the thickness of the photonuclear reactionconverter 16 cannot be increased arbitrarily, as it scatters and absorbsstrongly the heavy reaction products that arise. However, the reactionproducts can be increased by increasing the number of holes in thecollimator plate and/or by thinning the collimator, which, it is true,will result in a poorer position resolution ability. The total number ofreaction products is also affected by the intensity of the accelerator(60−>600 R/min, R=Röntgen=258 μC/kg), the radiation time, and the energy(=cross section area).

When using a photonuclear reaction converter 16, the interactions, i.e.the number of arising of ionizing recoil particles is independent of theamount (pressure) of the gas in the case 10. A larger electron avalanchecausing a charging impulse will be produced much more readily by a heavyparticle than by an electron scattered due to an interaction in the gas.Exploiting this allows the pressure and ignition voltage to beoptimized, to minimize the interference background caused by the Comptonelectrons arising in the gas and to attempt to emphasize the pulsesproduced by the recoil ions.

In the improved embodiment of the apparatus according to the invention,shown in FIG. 3, in the ionization chamber 10 are arranged twoionization detector planes 11.1, 11.2 acting as anodes are shown inconnection with the embodiment shown in FIGS. 1 and 2, while a thirddetector plane 12 acting as a cathode is located between them. Further,the photonuclear reaction converter 16 and the collimator 19 arearranged separately from the wall 10.1 that is a right angles to thedirection of the radiation.

Between the wall 10.1 and the photonuclear reaction converter 16, anadditional cathode plane 22 is arranged, corresponding functionally tothe said third detector plane 12, in connection with which means 13.4are arranged for conducting the current pulse t′ to the measurementelectronics. The use of the additional cathode plane 22 minimizes, amongother things, the interference background caused by the Comptonelectrons coming directly from the patient-care device, being scatteredby the environment, and arising in the wall 10.1. This is carried outwith the aid of the measurement electronics by compensating the signalst, t′.

In addition, the delay lines 17.1, 17.2 of the anode planes areconnected at both ends to conduct the current pulses X₁, X₂, Y₁, Y₂arising from the photonuclear reactions to the measurement electronics18. This improves the shape of the signals and the resolution ability ofthe XY position data of the ionization event. FIG. 3 also shows the gasfeed connection 20 arranged to the ionization chamber 10.

FIG. 4 shows an example of one preferred alternative for theimplementation of the measurement electronics, in the case of theembodiment shown in FIG. 3. The measurement electronics 18 comprisesfunctioning circuit components that are known as such in electronics ata component level.

The signal lines 13.3, 13.4 of the cathode planes 12, 22 include,itemized starting from the cathodes 12, 22, preamplifiers PA_(K1),PA_(K2) (e.g., KERT AT53S), which are followed by timing filteramplifiers TFA_(K1), TFA_(K2) (e.g., GSI DTFA 83). In the secondpreamplifier, the polarity of the signal is converted to the oppositesign. These components, which are thus located one on the signal lines13.1, 13.4 of the cathodes 12, 22, are followed by a common summingcircuit SUM for the lines 13.3, 13.4, by means of which the currentimpulses t, t′ brought from both cathodes are added together. Being ofopposite signs, they thus compensate each other.

Because the high-energy Compton electrons only interact slightly in thephotoelectron converter, the signals t, t′ that they cause compensateand only the signals t, which are caused by the reaction products, passthrough the SUM summing circuit. These are led to a constant fractiondiscriminator CFD_(K) (e.g., ORTEC CF 8000) and from there to adelay-gate generator CG_(K). After this, the cathode signal is led to ananalog to digital converter ADC (e.g., ORTEC 413A).

In order to lead the current pulses X₁, X₂, Y₁, Y₂ of the anode planes11.1, 11.2 to the measurement electronics 18, delay lines dedicatedseparate lines 13.1′, 13.1*, 13.2′, 13.2* are arranged for the outputsat both ends of the delay lines 17.1, 17.2. In the lines 13.1′, 13.1*,the anode 11.1 is followed by preamplifiers PA_(AX1), PA_(AX2), fromwhich the pulses X₁, X₂ are taken through constant fractiondiscriminators CFD_(AX1), CFD_(AX2) in a second line, for example,13.1′, directly to the time to amplitude converter TAC_(X) (e.g., ORTEC467).

In the second line 13.1*, before the TAC_(X) there is a delay gategenerator GG_(X2) and a delay element DL_(X2) (e.g., GAEN 107, GAEN108). This ensures that the signal X₂ is always delayed in relation tothe signal X₁ and that the pulse formed in the TAC_(X), which isproportional to their time difference, is positive. From the TAC_(X),the signal is led to the analog to digital converter ADC, as in the caseof the cathode lines 13.3, 13.4 described above.

The anode 11.2 that defines the ionization event in the Y direction hasalso a signal line arrangement that is functionally essentially similar,and has signal lines 13.2′, 13.2* starting from both ends of its delayline 17.2, in order to lead the current pulses Y₁, Y₂ to the measurementelectronics 18.

By means of the TAC_(X) and TAC_(Y), the time differences of the currentimpulses X₁, X₂, and Y₁, Y₂ are converted into amplitudes, the ADconversion of which can be interpreted as corresponding X, Y positiondata. By taking the current pulses from both ends of the delay lines17.1, 17.2, and deducting their values from each other, substantiallymore accurate XY position data is obtained than in the embodiment shownin FIG. 1, the measurement electronics 18′ of which otherwisecorresponds to that shown in FIG. 4, except that it lacks the signalline 13.4 corresponding to the additional cathode 22 and the summingcircuit SUM, as well as the signal lines 13.1*, 13.2*, equipped with,among other things, the delay elements DL_(X2), DL_(Y2), of the X and Yanode planes 11.1, 11.2. Operationally, this kind of implementation ofthe measurement electronics 18′ is functional, but by using the assemblyshown in FIG. 4 it is possible to achieve a considerably more accurateresult in defining the iXY position data of the ionization events of thephotonuclear reactions caused by the radiation field 23. An even betterresult than this can be achieve by applying the summing circuittechnique to the anode signals too.

The current pulses t, t′ from the cathodes 12, 22, converted by themeasurement electronics 18 and the current pulses of the anodes 11.1,11.2 are taken, for example, to a PC computer 24, in connection withwhich special software is arranged for calculating the XY position dataof the radiation field 23 as input from the given signals and in orderto present the shape of the radiation field 23.

Separate power supplies 26, 27 (typically 400–600 V) are arranged forthe cathode lines 13.3, 13.4 of the measurement electronics 18 and acommon bias power supply 25 is arranged for the anode lines 13.1′,13.1*, 13.2′, 13.2*. FIG. 4 does not show the actual power supplies ofthe anodes 11.1, 11.2 and the cathodes 12, 22.

FIG. 5 shows on a schematic level yet another embodiment of theimplementation of the photonuclear reaction converter 16. Because at lowgamma energies (less than 9 MeV) a converter 16 made purely from uranium238 will create practically no photonuclear reactions that are essentialfrom the point of view of the invention, the reaction converter 16 canbe arranged to comprise several layers on top of each other. In thisembodiment, the wall 10.1 of the ionization chamber 10 facing thearrival direction of the radiation field 23 is followed first of all bya beryllium layer Be-9, in which reactions arise already at energies ofless than 2 MeV (threshold energy 1,67 MeV). The Be-9 layer is followedby a uranium layer U-238. In FIG. 5, the layers are shown as beingseparate from each other, to better depict the reaction productscreated. In reality, the layers thus lie against each other.

The radiation 23 first causes a Be-9 gamma −>neutron Be(y, n) reactionin the beryllium layer, in which two alpha particles are released inaddition to neutrons. The neutrons released in the reaction next strikethe uranium layer U-238, causing a neutron −>fission U(n,f) reaction init. In this case, the released heavy fission products strongly ionizethe gas, which is measured in the same way as has already been describedin the gamma −>fission U(y, f) reaction that takes place at higherenergies.

FIG. 6 [1] shows the neutron cross-section areas in uranium U-235,U-238, and plutonium Pu-239. It can be seen from the figure, that atless than 2 MeV the neutrons no longer create reactions in U-238, thematerial of the converter having to be replaced by U-235 (Pu-239). Theuse of other isotopes of uranium and plutonium is naturally alsopossible.

Next, the operation of the apparatus according to the invention isdescribed. A photon (gamma beam) emitted by the patient-care device (notshown) creates a photonuclear reaction in the uranium or beryllium layer16 of the ionization chamber 10, the heavy nuclei or alpha particlesthat arise in connection with which ionize the gas in the chamber 10very strongly. In the reaction, in addition to heavy nuclei or alphaparticles, either one or more light particles, typically neutrons arise,so that it is a question of a so-called photoneutron reaction, oralternatively of two medium-heavy nuclei, in which case the question isof so-called photofission. When the gas is ionized, an electron isdetached from it, which, when it travels through the ionization chamber10, collides with the electrons of other gas atoms and thus creates anelectron avalanche, which is sufficiently large to create a currentpulse measurable using the detector devices 12, 22, 11.1, 11.2.

There is a difference in potential between the cathodes 12, 22 and theanodes 11.1, 11.2 of the ionization chamber 10, so that the electronavalanche created travels to the anodes 11.1, 11.2 in the positivepotential. The electron avalanche causes a current pulse detectable withthe measurement electronics 18, in those wires 14.1, 14.2 that arelocated mainly close to the point hit by the gamma beam in the converter16. As a result of the ionization, the gas molecules and ions create acurrent pulse t in the cathode 12, which is used to set the start of thetime window for the measurement electronics 18. The current pulses t canbe differentiated, as the pulses that take place simultaneously with thecurrent pulses registered by the additional cathode 22 are left out ofconsideration. After the initial point t, the corresponding points intime, by means of which the XY position data of an ionization event canbe determined, are determined using the measurement electronics 18 forthe current pulses X₁, X₂, Y₁, Y₂, corresponding to the XY position ofthe ionization, arriving at the measurement electronics 18 from theanode planes 11.1, 11.2.

In reality, the electron avalanche that travels to the anodes 11.1, 11.2may cause a current pulse in several adjacent anode wires 14.1, 14.2.However, this has no significant effect on the measurement accuracy ofthe apparatus, as the signals of the wires adjacent to the wirecorresponding to the real hit point are sufficiently weak for theposition data to be essentially ascertained to an accuracy of a singlewire.

The current pulse is collected by the anodes 11.1, 11.2 to the delaylines 17.1, 17.2, in which the speed of travel of the charge, i.e. thedelay time of the delay elements 15 forming the delay lines 17.1, 17.2,is known. Compared to this, the delays of the individual anode wires14.1, 14.2 are so small that they have no relevant significance. If, forexample, in the case of the apparatus shown in FIGS. 1 and 2, it takes100 ns for the charge to travel from one end of a delay line 17.1, 17.2to the other, and if it takes 70 ns for the charge to travel from thehit point on the delay line 17.2 defining the X direction and 30 ns totravel from-the hit point on the delay line 17.1 defining the Ydirection, the hit position can be determined using the measurementelectronics 18 in the defined set of XY co-ordinates. In this case, itis the point (T_(X)=70, T_(Y)=30). By collecting a group of points(T_(X), T_(Y)), it is possible to determine the intensity distributionin the surface of the ionization chamber 10.

In the embodiment shown in FIG. 3, the times corresponding to thecurrent pulses X₁, X₂, Y₁, Y₂ obtained from both ends of the delay lines17.1, 17.2 are deducted from each other X₂−X₁, Y₂−Y₁, giving a sharpersignal, on the basis of which substantially more precise XY positiondata is obtained. The time difference of the current pulses isproportional to the position data of the ionization event.

Due to the short measurement time achieved using the apparatus accordingto the invention, changes in the ambient temperature taking place duringthe measurement event and the changes in the pressure of the gas in theionization chamber 10 caused by them need not be taken into account. Inaddition, the XY positioning accuracy of the measurement points isexcellent, as is it determined only by the mutual distance between thewires 14.1, 14.2 forming the anode planes 11.1, 11.2.

An accurate gas feed, pressure regulation, and monitoring system (notshown) can also be fitted to the apparatus according to the invention,which will also permit pressure tests to be made. The gas pressure inthe apparatus is optimized according to the heavy ions, so that neithertoo many nor too few excitations take place. The measurement electronics18 can be adapted to allow pulse height and threshold voltage to beexploited.

The apparatus includes the necessary connections for the measurementelectronics 18, gas-, feed, and data processing, as well as a controland operating system. The systems include accurate software-based,automatic control possibilities for monitoring the ignition voltage andthe gas pressure. The apparatus includes not only the measurementelectronics 18, but also an user interface 30 for controlling them andfor processing the measurement data. The user interface 30 can be easilyconnected to an automatic data system serving quality control and to theaccelerator (not shown).

Neutrons too are produced in the photonuclear reactions. In practice,there are, however, very few of them. The walls of the ionizationchamber 10 can be constructed from a material that slows down neutrons,for example, by lining them with paraffin wax or boron. In addition,when the accelerator is operating, no-one is allowed to be in themeasuring room anyway, so that the exploitation of the photonuclearreactions is detrimental to neither the apparatus operators nor to theenvironment. Inside the ionization chamber 10, the thermic neutrons canproduce long-term activity only in the thinly-shaped photonuclearreaction converter 16 and the anode wires 14.1, 14.2, but due to thesmall amount of heavy materials this is not significant. However, thereis reason to monitor the possible accumulation of activity.

One important advantage achieved by the invention is the elimination ofpossible human errors in dosimetric measurements. This is achieved bythe user-friendly and easy-to-use operating system 30 and by integratingthe accelerator too in the measurement. In addition, the measurementprocess becomes simpler and more rational, as the apparatus according tothe invention eliminates unnecessary manual work stages.

The following is a collection of the more important measurementsrelating to radiotherapy quality control, which can be advantageouslyperformed using the apparatus according to the invention.

Isodose measurements, in which the dose distribution in water atstandard depths of a radiation field is measured using predefinedfield-size values, according to a specific measurement protocol. Themeasurement results can be used to configure a dose-planning program andfor quality control.

The definition of field-size coefficients and wedge coefficients, whichis carried out for predefined rectangular fields. In this case, the dosemaximum on the centre axis of the radiation is defined and the doseproduced by a specific monitor-unit amount (MU) at the location inquestion is measured. The results are standardized against the dose of astandard field (10 cm×10 cm). The wedge coefficients are defined inrelation to the doses of an open field and a corresponding wedge field.

Dose calibration, in which the correspondence between dose produced bythe care device and the monitor-unit setting of the device isdetermined. The dose is defined in a dose maximum for each type ofenergy and radiation, in the centre axis of a field with a size of 10cm×10 cm. The accuracy of the dose calibration is extremely important,because it forms the basis of all the care doses.

In determining the dose linearity of the linearity of a dose during theradiation of a predefined monitor-unit amount.

Determining dose repeatability, in which radiation of a specificmonitor-unit amount is repeated several times and the repeatability ofthe dose is determined.

The dependency of the dose on various angles of tilt, because theradiation produced by a radiotherapy accelerator depends to some extenton the angle of tilt of the accelerator. The variation is cause by themechanism of the accelerator and to some extent by gravity.

Due to the tolerances in the moving parts of the accelerator and othermechanical constructions variations in the dose also appear as afunction of the rotation of the collimator. The dose variation ismeasured using a predefined collimator angles at all energies.

It must be understood that the above description and the related figuresare only intended to illustrate the present invention. The essentialelement in the method and apparatus according to the invention is theuse of photonuclear reactions. On the basis of the description, thereare indeed several alternatives for the measurement electronics and themanner of determining the XY position data of the ionization event.Thus, the invention is not restricted only to the embodiments presentedabove or defined in the claims, but instead many different variationsand adaptations that are possible within the scope of the inventive ideadefined by the accompanying Claims will be obvious to one versed in theart.

References:

[1] OECD/NEA 1989, Plutonium fuel—an assessment. Taube 1974, Plutonium—ageneral survey.

1. A method for determining the intensity distribution of a radiationfield, in which the ionization caused by the radiation field is detectedby means of parallel ionization detector planes arranged in anionization chamber, two of which planes are formed of series of wiresdetermining the position of the ionization event in the X and Ydirections, which detector planes provide the X and Y co-ordinates X₁,Y₁ of the ionization event, characterized in that the ionization eventis created by means of the photonuclear reaction products arising fromthe radiation.
 2. A method according to claim 1, characterized in thatthe direction of travel of the photonuclear reaction products iscollimated to be at essentially right-angles to the detector planes. 3.A method according to claim 1, characterized in that the saidphotonuclear reaction products are created through one or severalintermediary product stages.
 4. An apparatus for determining theintensity distribution of a radiation field, which apparatus includes atleast two parallel ionization detector planes arranged in an ionizationchamber, which are arranged to be formed of series of wires determiningthe position of an ionization event in the X and Y directions and thatthey are arranged to provide the X and Y co-ordinates X₁, Y₁ of theionization event, characterized in that, in addition, a photonuclearreaction converter is fitted to the ionization chamber, in order toachieve indirect detection in the apparatus.
 5. An apparatus accordingto claim 4, characterized in that the said photonuclear reactionconverter is arranged to include one or more layers.
 6. An apparatusaccording to claim 4, characterized in that the thickness of the saidphotonuclear reaction converter is 0.1–100 μm.
 7. An apparatus accordingto claim 4, characterized in that the said photonuclear reactionconverter is arranged in the ionization chamber in connection with thewall arranged facing the direction of the radiation field.
 8. Anapparatus according to claim 4, characterized in that collimator devicesfor controlling the direction of travel of the photonuclear reactionproducts are arranged in connection with the apparatus.
 9. An apparatusaccording to claim 4, characterized in that the apparatus includes athird detector plane for determining the time window of each measurementand that the said two detector planes are arranged to act as anodes andthe said third detector plane as a cathode.
 10. An apparatus accordingto claim 4, characterized in that the series of wires of the said twodetector planes are arranged to be connected at least at one end to themeasurement electronics.
 11. An apparatus according to claim 10,characterized in that the series of wires of the said two detectorplanes are at one end arranged to be connected by delay elements, sothat the said delay elements connected consecutively are arranged tocreate a delay line, by means of which the current pulse generated bythe ionization event is arranged to be led to the measurementelectronics from at least one end of the delay lines.
 12. An apparatusaccording to claim 11, characterized in that a current pulse generatedby the radiation field is arranged to be led from both ends of the delaylines to the measurement electronics, in which delay means (DL_(X2),DL_(Y2)) are arranged for the current pulses led from one end of thedelay lines, in order to improve the position resolution of theapparatus.
 13. An apparatus according to claim 4, characterized in thatthe said two detector planes are arranged to be formed of series ofwires, the distance between wires within the detector planes is arrangedto be 1–10 mm.
 14. An apparatus according to claim 9, characterized inthat, in addition to the said detector planes, a plane correspondingfunctionally to the said third detector plane is arranged in theionization chamber, in order to improve the resolution capability of theapparatus.
 15. An apparatus according to claim 4, characterized in thatthe ionizing gas arranged in the ionization chamber is one of an inertgas, carbon hydride, a compound based on an inert gas, and a compoundbased on carbon hydride.
 16. An apparatus according to claim 5,characterized in that the layers of the said photonuclear reactionconverter are uranium.
 17. An apparatus according to claim 5,characterized in that the layers of the said photonuclear reactionconverter are beryllium.
 18. An apparatus according to claim 5,characterized in that the layers of the said photonuclear reactionconverter are uranium and beryllium.